1. Field of the Invention
The present invention relates to a liquid crystal display device of optical writing type.
2. Description of the Related Art
As one of various kinds of liquid crystal display devices, there is a liquid crystal display device of optical writing type, which is typically used as a liquid crystal light valve.
Such a liquid crystal display device of optical writing type includes a pair of glass substrates facing each other.
Each of the substrates is provided with a transparent electrode on the facing side.
On the first transparent electrode, there is formed a photoconductive layer. The photoconductive layer is made from hydrogenated amorphous silicon (a-Si:H).
On the photoconductive layer, there is formed a dielectric mirror layer. The dielectric mirror layer is made from multiple layer films composed of Si (silicon)/SiO.sub.2 (silicon dioxide), TiO.sub.2 (titanium dioxide)/SiO.sub.2, or ZnS (zinc sulfide)/MgF (magnesium fluoride), etc.
Between the photoconductive layer and the dielectric mirror layer, there may be interposed a light absorbing layer. The light absorbing layer is made from a patternized thin film including C (carbon), Ag (silver) and so on.
A pair of orientation films are formed on the dielectric mirror layer and the second transparent electrode. A liquid crystal layer is disposed between the orientation films and sealed by use of a sealing member which functions as a spacer and attaches the glass substrates to each other.
Such a liquid crystal display device is used as a liquid crystal light valve, for example, for use in a liquid crystal projector.
In the operation of the liquid crystal display device as a light valve, an alternating electrical voltage is applied across the transparent electrodes. Then, a laser beam scans the substrate from the side of the photoconductive layer so as to change the impedance of the photoconductive layer, change the voltage applied to the liquid crystal layer, and change each molecular orientation of the voltage applied area of the liquid crystal layer. Thus, an image due to the impedance differences of the photoconductive layer is achieved on the liquid crystal layer, depending on the condition of the photoconductive layer, i.e., whether it is in either a dark condition (where the laser beam is applied) or a bright condition (where the laser beam is not applied).
In case of utilizing such a light valve for the liquid crystal projector, the written image on the liquid crystal layer is projected onto a screen by use of a projection light from a light source.
As an operation mode in such a display operation, there are a TN (twisted nematic) mode, a HFE (hybrid field effect) mode, a GH (guest host) mode, a phase transition mode and so on.
In this kind of liquid crystal display device of optical writing type, since the hydrogenated amorphous silicon layer is used as the photoconductive layer, the conductivity of the photoconductive layer in the dark condition is of the same order as the conductivity of the liquid crystal layer, which is about 10.sup.-10 to 10.sup.-12 S/cm. The impedance of the photoconductive layer and the liquid crystal layer are in a same order. Accordingly, on one hand, in the dark condition, a certain amount of the voltage is applied to the liquid crystal layer. On the other hand, in the bright condition, the impedance of the photoconductive layer is reduced to be lower than the impedance of the liquid crystal layer, so that substantially the entire voltage is applied to the liquid crystal layer. In this structure, the ratio referred to as the ON/OFF voltage ratio, which is the ratio of the voltage applied to the liquid crystal in the area where light is applied to the voltage applied to the liquid crystal in the area where light is not applied, is lowered because a certain amount of the voltage is applied to the liquid crystal layer even in the dark condition, resulting in an increase of the OFF voltage. Consequently, a first problem areas in this kind of liquid crystal display device is that a high contrast image can not be obtained.
In this kind of liquid crystal display device, in the case wherein a light absorbing layer is utilized, since the light absorbing layer is formed from a metal film including C, Ag, etc., and since the dielectric mirror layer is made from multiple layers of Si/SiO.sub.2, TiO.sub.2 /SiO.sub.2, ZnS/MgF, etc. as described above, the resolution of the image liquid crystal display device is rather poor, giving rise to a second problem in this kind of display device.
In addition, the kind of liquid crystal display device has a third problem, that is, since the adhesion ability between the metal film including C, Ag, etc. and the hydrogenated amorphous silicon is poor, the light absorbing layer and the photoconductive layer are easily detached from each other.
Further, the manufacturing process of the dielectric mirror layer is rather difficult and complicated, since the production of the multiple layer structure requires repeating two different processes by turns, i.e. changing the material and forming each film from the changed material in each step, giving rise to a fourth problem.